Abstract
This work presents an investigation into the behaviour of Polyetheretherketone (PEKK)/carbon thermoplastic composite joints, addressing the relatively low joint strengths that affect their suitability for use in load-bearing, structural applications. The current resurgence in interest in thermoplastic composites, due to their advantageous attributes, such as anhygroscopity, potential recyclability, capacity for secondary thermoform joint formation and superior damage tolerance, means that the development of thermoplastic composite joints that are suitably strong for use in structures has become highly relevant. This work determines how joint consolidation methods, including both adhesive bonding and thermoforming, can be developed to improve joint performance.A review of the literature reveals that the knowledge surrounding the behaviour of thermoplastic joints, in particular, the implementation of joint strengthening methodologies, have been largely focussed on the surface chemistry effects and adhesive behaviour in peel. Additionally, previous work documenting the effect of novel thermoforming methods, including the inclusion of additional polymer layers, has not only included limited work on static behaviour and no work on fatigue or damage.
Functionalising surface treatments, including both ultra-violet generated ozone (UV/O3) and atmospheric plasma discharge, are used to modify PEKK/carbon composite surface characteristics and improve joint bondline adhesion and thus joint strength. A design of experiment approach is implemented to better understand the significance of individual surface treatment factors and their interactions on joint strength. The static and dynamic performance of thermoformed PEKK/carbon joints is examined along with the damage tolerance of both the highest performing adhesively bonded and thermoformed joints.
Both UV/O3 and atmospheric plasma surface treatments improve joint lap shear strength (LSS) by 18.2 ± 6% and 22.2 ± 1.9% respectively. The relatively short post-exposure hydrophobic recovery period, observed with UV/O3 treatment, can potentially be increased using a higher treatment temperature. Atmospheric plasma treatment efficacy can be improved through intermittent recovery time during treatment, achieved by using a higher number of plasma nozzle passes. The static strength (LSS) of both directly thermoformed and embedded polyetherimide (PEI) interlayer thermoformed joints is statistically indistinguishable. However, the performance in both low and high cycle fatigue of the embedded PEI joints is superior, achieving 2.5 and 4.7 times greater fatigue lives respectively, showing better damage tolerance mechanisms during testing, including a relatively high degree of plasticity. Under impact, the embedded PEI interlayer joints were again superior, showing a far greater capacity to absorb impact energy (31 ± 1.1%) and retain pristine static strength (LSS), than both the directly thermoformed and plasma treated adhesively bonded joints, which absorbed 22 ± 0% and 8.5 ± 2.4% respectively.
In conclusion, the performance of PEKK/carbon composite joints can be improved through surface functionalisation using either UV/O3 or atmospheric plasma, whose treatment efficacy can be improved using increased treatment temperature and increasing the number of nozzle passes, respectively. Atmospheric plasma generated the strongest adhesively bonded joints. The performance of thermoformed joints can be improved using an embedded PEI interlayer, which reduces process temperature requirements and cycle time while improving fatigue performance and damage tolerance. Joints that had been thermoformed using an embedded PEI interlayer yielded the strongest and most damage tolerant lap joint configuration.
| Date of Award | Dec 2019 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Brian Falzon (Supervisor) & Damian Quinn (Supervisor) |